The use of heterogeneous, ion-exchange resins as catalysts was recognized before the development of modern styrenic resins. Cottle (U.S. Pat. No. 2,678,332; issued 11 May 1954, filed 23 Dec. 1949) employed sulfonated phenol-formaldehyde resins and even sulfonated coal for esterification, in which the reactants were an olefin with an organic acid. Indeed, there had been work at I.G. Farbenindustrie, BASF, in Germany during World War II using resins as catalysts [DE 882,091 (1942); DE 866,191 (1944), DE 868,147 (1944)] but these were not published until around 1952.
Universally, there are three target properties for any commercially-useful catalyst: (a.) increased activity, (b.) reduced by-product formation, due to side and/or further reaction(s), and (c.) longer catalyst lifetimes leading to (d.) longer process runs. Aries dealt with the first two of these issues: U.S. Pat. No. 3,053,887 (issued 11 Sep. 1962, filed 3 Nov. 1959) describes the alkylation of carboxylic acids with olefins, to produce tertiary-alkyl esters. Resins of low porosity (i.e., styrenic gel resins) were not effective catalysts, but styrenic macroporous resins (e.g., Duolite C-25) were. And, Aries succeeded in suppressing the well-known, competitive polymerization of the olefin (iso-olefin, isobutylene: self alkylation): having found effective catalysts, he was then able to carry out the reaction at lower temperatures.
Five factors are crucial for effective reaction:                A. compatibility of the reagents—e.g., when trying to contact a very hydrophilic species with a very hydrophobic one (a classic example being aqueous cyanide ion with a larger alkyl halide),        B. compatibility of the catalyst or catalytic sites with all the required reagents for reaction—i.e., solubility of the reagents in the microenvironment of the catalytic sites,        C. accessibility of the catalytic sites to all the required reagents: the rates of diffusion of the reagents to the catalytic sites—e.g., large molecules versus small ones, the available surface area of the catalyst, and the crosslinking in the gel region of a resin,        D. permeation of the product(s) out of the catalyst,        E. and obviously, the activity of the catalytic group itself—e.g., strong versus a weak acids.Conceptually, factor A. can be subsumed into B. when the catalysis is heterogeneous.        
In the esterification of carboxylic acid with alcohols rather than olefins, another side reaction, ether formation, can prove troublesome.
Similar side reactions can occur in transesterification: (1) ether formation, (2a) dehydration to olefin(s), as well as (2b) polymerization thereof.
A number of researchers have attempted to provide improved, useful heterogeneous catalysts. For example, U.S. Pat. No. 3,678,099 (Kemp, issued 18 Jul. 1972, filed 26 Jun. 1970) describes an IEX catalyst exchanged with metal salts for olefin esterification, as did U.S. Pat. No. 3,278,585 (Baker, issued 11 Oct. 1966, filed 8 Aug. 1962) for alcohol esterification. WO98/25876 (Young, Int. Pub. Date of 18 Jun. 1998, priority of 12 Dec. 1996) describes an IEX catalyst for gas phase esterification. U.S. Pat. No. 4,332,738 (Benitez, issued 1 Jun. 1982, filed 24 Nov. 1980) discloses a macroreticular (i.e., macroporous) IEX catalyst for the esterification of neo acids with alcohols; and U.S. Pat. No. 4,698,186 (Jeromin; issued 6 Oct. 1987, filed 21 Jan. 1986) describes IEX catalysts for pre-esterification of acids in fats. However, each of these catalysts are limited. For example, some of these catalysts are useful for only a limited range of materials, a limited range of reaction conditions, and still too much side products or by-products are formed, or the catalyst is not sufficiently stable, long-lived, and/or not economically effective.
U.S. Pat. No. 5,426,199 (Lundquist, issued 20 Jun. 1995, initially filed 13 Dec. 1991) describes the catalyzed esterification of an alcohol with an acid or an ester (transesterification). The esterification process involves contacting an organic acid or ester with an alcohol in the presence of crosslinked, vinylaromatic copolymer beads which have an inner volume of unfunctionalized polymer and a surface layer functionalized with strongly acidic functional groups. Such resins have been denoted as having a “shell-core” morphology. The total cation-exchange capacity is from about 0.1 to about 2.5 meq/gdry which, taking into consideration the ethylvinylbenzene in commercial divinylbenzene, calculates to ca. 1 to 33% functionalization. The resin catalysts are produced from gel or macroporous copolymers by sulfonating, according to the previous U.S. Pat. No. 3,242,921 (Hansen, issued 24 May 1966, filed 18 Mar. 1965), only the surface layer.
Lundquist's theorized that with fully sulfonated resins, the surface groups are accessible to essentially all reactants, while those deeper within the polymeric matrix are accessible only to (a) small, (b) polar reactants. Therefore, small, polar, organic alcohols partition themselves within the interior of the resin matrix and preferentially form (1) ethers through self condensation and/or (2) olefinic by-products through dehydration. Widdecke [“Design and Industrial Application of Polymeric Acid Catalysts,” Chapter 4, p. 166; D. C. Sherrington and P. Hodge, editors, Syntheses and Separation using Functional Polymers, John Wiley & Sons Ltd., Chichester (1988)] had also used a surface sulfonated resin. However, Widdecke taught that: the reaction of the non-polar reagent will be restricted mainly to the surface active groups, while the undesired reaction (formation of dimethyl ether from methanol) will take place within the microspheres or “microphase.”
The need for catalysts that produce less and/or fewer by-products has been recognized in the art for a goodly time, now. Lundquist's approach addressed incompatibility or partitioning effects. However, there are other issues or factors, such as fouling, thermal de-sulfonation, acidity, crosslinking and its effects, etc. Additionally, there is need for good, effective methods to produce such resins, efficiently, economically, and environmentally.
All U.S. patents and publications cited herein are hereby incorporated by reference. In the event of differing terminology or disclosure, that of the present specification controls.